The present invention relates generally to soft magnetic materials. In particular, the present invention relates generally to soft magnetic materials used in various electromagnetic devices. More particularly, the invention relates to soft magnetic materials and composite magnetic articles made of coated ferromagnetic particles.
Magnetic materials fall generally into two classes, hard magnetic materials which may be permanently magnetized, and soft magnetic materials whose magnetization may be reversed. The present invention relates to the latter class of materials. The magnetic permeability and core loss characteristics are important properties of soft magnetic materials in electromagnetic applications. Magnetic permeability is a measure of the ease with which a magnetic substance may be magnetized and is an indication of the ability of the material to carry a magnetic flux. Magnetic permeability is defined as the ratio of the induced magnetic flux to the magnetizing force or the magnetic field intensity. The exposure of a magnetic material to a rapidly varying field results in an energy loss in the magnetic core of the material, which energy loss is known as the core loss. Core loss is divided into two categories, hysteresis loss and eddy current loss. The hysteresis loss results from the expenditure of energy to overcome the retained magnetic forces in the magnetic core. The eddy current loss results from the flow of electric currents within the magnetic core induced by the changing flux.
Conventional electromagnetic devices use magnetic core articles made using laminated structures. Laminated cores are typically made by stacking thin ferrous sheets which are oriented parallel to the magnetic field to provide low reluctance. The sheets may be coated to provide insulation and prevent current from circulating between sheets. Such insulation results in a reduction in the eddy current loss. The fabrication of laminated cores involves many operations which contribute to increased expense. The application of laminated cores is limited by the need to carry magnetic flux in the plane of the sheet to avoid excessive eddy current losses. The fabrication of three-dimensional configurations using the lamination process is expensive and complex. Laminated cores experience large core losses at high frequencies and are acoustically noisy as the laminations have a tendency to vibrate. The use of sintered and coated ferromagnetic powders for making magnetic core articles allows greater variation in the geometry of the component and avoids the manufacturing burden inherent in laminated cores. However, magnetic core articles made using sintered ferromagnetic powders experience high core losses and typically have been restricted to applications involving DC operation.
The use of encapsulated ferromagnetic powders to make magnetic core articles has been and continues to be a subject of research. The encapsulation provides an electrical insulation for individual ferromagnetic particles to reduce eddy current losses and may also serve as a binder or a lubricant. The desired properties in magnetic core articles made using encapsulated ferromagnetic powders include high density, high permeability, low core losses, high transverse rupture strength, and suitability for compaction molding techniques. Various attempts have been made to form magnetic core articles using encapsulated ferromagnetic powders. Several types of encapsulating materials and encapsulating methods have been used. Inorganic encapsulating materials such as iron phosphate, iron chromate, iron oxides and boron nitride have been suggested. Certain organic encapsulating materials have also been used. Doubly encapsulated ferromagnetic powders have also been suggested for making magnetic core articles. Encapsulating materials made by blending different materials have also been suggested.
The encapsulated ferromagnetic powders are compacted into a magnetic core article. Following compaction, the properties of magnetic core articles, made using such encapsulating materials and the suggested encapsulating methods, such as the permeability and core losses are less than desired particularly at low frequency operation. Annealing the magnetic core article can result in increased permeability and lower core loss. Annealing relieves residual stresses caused by compaction of the encapsulated ferromagnetic powders. These residual stresses degrade magnetic properties such as permeability and core loss characteristics. In order to achieve an effective anneal and substantially relieve the residual stress, the article is maintained at a temperature typically in excess of 600xc2x0 C. for a duration that depends on the extent of residual stress present. However, a temperature approaching 600xc2x0 C. causes most organic encapsulating materials to degrade, decompose, or pyrolyze. This impairs the ability of the encapsulating material to electrically insulate the ferromagnetic powders and results in degradation of the permeability, core loss, and mechanical integrity of the magnetic core article.
Therefore, there exists a continued need to produce coated ferromagnetic particles and magnetic articles comprising coated ferromagnetic particles having high permeability and low core loss characteristics in a cost effective manner.
An embodiment of the present invention provides a coated ferromagnetic particle. A coated ferromagnetic particle in accordance with one embodiment of the invention comprises a ferromagnetic core and a coating. The coating comprises a residue resulting from a thermal treatment of a coating material comprising a polymer selected from the group consisting of polyorganosiloxanes, polyorganosilanes, and mixtures thereof.
Another embodiment of the invention provides a composite magnetic article comprising a compacted and annealed article of a desired shape. The composite magnetic article comprises a plurality of coated ferromagnetic particles. Each coated ferromagnetic particle comprises a ferromagnetic core and a coating. The coating comprises a residue resulting from a thermal treatment of a coating material comprising a polymer selected from the group consisting of polyorganosiloxanes, polyorganosilanes, and mixtures thereof.
In another embodiment of the present invention, a method for making a coated ferromagnetic particle comprises the steps of: (a) providing an uncoated ferromagnetic core; (b) providing a coating material comprising a polymer selected from the group consisting of polyorganosiloxanes, polyorganosilanes, and mixtures thereof; (c) encapsulating the uncoated ferromagnetic core with the coating material comprising the polymer; and (d) thermally treating the coating material so as to convert the coating material into a residue.
Still another embodiment of the present invention provides a method for producing a composite magnetic article. The method for producing a composite magnetic article comprises the steps of: (a) providing uncoated ferromagnetic particles; (b) providing a coating material comprising a polymer selected from the group consisting of polyorganosiloxanes, polyorganosilanes, and mixtures thereof; (c) encapsulating each of the uncoated ferromagnetic particles with the coating material to produce encapsulated ferromagnetic particles; (d) subjecting the encapsulated ferromagnetic particles to a compaction to form a compact of a desired shape; and (e) subjecting the compact to an annealing treatment. The composite magnetic article comprises a plurality of coated ferromagnetic particles. Each of the coated ferromagnetic particles comprises a ferromagnetic core and a coating. The coating comprises a residue resulting from the thermal treatment of a coating material comprising a polymer selected from the group consisting of polyorganosiloxanes, polyorganosilanes, and mixtures thereof.
According to another aspect of the invention, a device using electromagnetic materials comprises a composite magnetic article.
These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description, appended claims, and accompanying drawings in which like characters represent like parts throughout the drawings.